PHP 8 Support

[reformed]

This library is causing a deprecation notice:

Deprecated: Return type of Suin\RSSWriter\SimpleXMLElement::addChild($name, $value = null, $namespace = null) should either be compatible with SimpleXMLElement::addChild(string $qualifiedName, ?string $value = null, ?string $namespace = null): ?SimpleXMLElement, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in Suin\RSSWriter\SimpleXMLElement.php on line 17

The Suin\RSSWriter\SimpleXMLElement.php file should be updated such that line 17:

public function addChild($name, $value = null, $namespace = null)

should become:

public function addChild(string $qualifiedName, ?string $value = null, ?string $namespace = null): ?static